The present invention relates to a new formulation of fuel and a process for making such fuel. More specifically, the fuel of the present invention is an energy fortified diesel fuel which is useful in off-highway applications.
Diesel engines having different fuel requirements can be grouped into three main categories, namely, on-highway compression ignition engines, off-highway heavy duty compression ignition engines, and off-highway "very" heavy duty compression ignition engines. Off-highway heavy duty compression ignition engines operate at lower speeds and have fewer load changes than on-highway compression ignition engines. An example of such an engine is a locomotive engine. Off-highway "very" heavy duty compression ignition engines operate in the least demanding environment of these three categories of engines. They operate at slower speeds and have fewer load changes than off-highway heavy duty compression ignition engines. Marine diesel engines and electric power generation engines are examples of off-highway "very" heavy duty compression ignition engines.
Currently, in choosing a diesel fuel for off-highway heavy duty compression ignition engines, one must choose between two non-optimum extremes. One extreme is on-highway diesel fuel, which has good compression ignition behavior, as indicated by its high cetane number (or equivalently, its cetane index, which is more commonly measured).
The cetane number of a diesel fuel is determined by use of a laboratory single cylinder compression ignition engine by comparison of the compression ignition delay of the sample fuel with blends of cetane and heptamethylnonane, where cetane (n-hexadecane, C.sub.16 H.sub.34) is arbitrarily assigned a number of 100 and heptamethylnonane is assigned a number of 0. The cetane number method is published as ASTM D 613: Test Method for Cetane Number of Diesel Fuel Oil. Alternatively, the cetane index (ASTM D976 Test Method for Calculated Cetane Index of Distillate Fuels) gives a good approximation of the cetane number based on the distillation and density of the diesel fuel.
In the United States, the Environmental Protection Agency (EPA) specifies that on-highway diesel fuel have a cetane index of at least 40. European standards require the cetane number for on-highway diesel fuel to be a minimum of 50. On-highway diesel fuel is further characterized by a high API gravity and, accordingly, a low specific gravity and a relatively low volumetric energy density (Btu/gallon). All API gravity and volumetric energy density numbers throughout this patent application are measured at about 60.degree. F. and about 1 atm. Specifically, on-highway diesel fuel has an API gravity of about 30 or greater and a volumetric energy density of about 133,000 Btu/gallon or less. Much of on-highway diesel fuel has an API gravity greater than 35 and a volumetric energy density less than 130,000 Btu/gallon. Still further, at least 90% of on-highway diesel fuel must vaporize at or below 640.degree. F. according to standards set by the American Society of Testing and Materials (ASTM). At least 95% of on-highway diesel fuel must vaporize at or below 698.degree. F., according to European standards. One example of an on-highway diesel fuel is one that meets ASTM D 975 standards. Percentages referred to in this patent application are expressed in terms of volume percent unless otherwise noted.
The high cetane number required for on-highway diesel fuels is not necessary for off-highway heavy duty and "very" heavy duty compression ignition engines. For instance, according to the American Association of Railroads (AAR), locomotive diesel engines can successfully operate with fuels having cetane numbers as low as 32 or lower. Also, AAR studies indicate that diesel fuel used in locomotive engines can have a higher distillation range than that specified by ASTM for on-highway diesel fuel. Thus, using on-highway diesel fuel for off-highway applications results in unnecessary fuel costs since on-highway diesel fuel does not provide benefits to off-highway compression ignition engines by having a higher cetane number or lower distillation range than necessary for off-highway operation. Still further, as determined by experiments in developing the present invention, the high API gravity of on-highway diesel fuel is not necessary for off-highway applications.
The other non-optimum extreme for use in off-highway compression ignition engines is bunker fuel or bunker fuel blends. Bunker fuel is the bottom of the barrel product in a refinery. It has a very high boiling range including a substantial portion boiling higher than 900.degree. F. It has a low API gravity, which is typically about 10, and thus, it has a high volumetric energy density of about 145,000 Btu/gallon. Because of its extremely high volumetric energy density, it provides excellent fuel economy.
Bunker fuel is conventionally used in "very" heavy duty compression ignition engines, such as marine diesel engines, which run at low speeds with relatively steady load requirements. However, bunker fuel or even blends of bunker fuel with typical on-highway diesel fuel is not suitable for on-highway applications and higher speed off-highway applications, such as off-highway heavy duty compression ignition engines like locomotive engines, which operate at higher speeds and have more transient load requirements than "very" heavy duty compression ignition engines.
The disadvantages caused by bunker fuel and bunker fuel blends when used in on-highway compression ignition engines and off-highway heavy duty compression ignition engines outweigh the fuel economy provided in these applications. These disadvantages derive from its formulation since certain impurities found in bunker fuel, such as ash (non-combustibles), asphaltenes, particulates (insolubles), Ramsbottom carbon (correlated with Conradson carbon), metals, high boiling compounds, and non-vaporizable components, can be detrimental to performance and durability. The relatively high quantities of ash found in bunker fuel result from compounds which dissolve in the heavy oil fractions of the bunker fuel or tramp contaminants that enter the fuel during processing from the well to the end user. Particulates and gums, which are also found in bunker fuel in relatively high quantities, collapse fuel filters and cause filter and line plugging. Still further, bunker fuel has a high Ramsbottom carbon value, as compared with typical on-highway diesel fuel, which has a negligible amount of Ramsbottom carbon. Bunker fuels also have relatively high viscosity, and therefore, their atomization in compression ignition fuel injectors is less efficient than lower viscosity diesel fuels.
The compression ignition behavior deficiencies caused by bunker fuels do not preclude their application for the "very" heavy duty engines because of the unique speed and load requirements of these engines. However, for on-highway compression ignition engines and off-highway heavy duty compression ignition engines that operate at higher speeds and experience transient load requirements as a significant part of their duty cycles, bunker fuels do not provide acceptable compression ignition behavior, and they can form relatively large amounts of soot and otherwise give unacceptable performance, such as compromising durability and shortening the life of engine components. Thus, in choosing between using on-highway diesel fuel and bunker fuel for off-highway heavy duty compression ignition engines, typically, on-highway diesel fuel is chosen.
Still further, on-highway diesel fuel has been blended with light cycle oil (LCO) wherein greater than about 50% of the LCO vaporizes at temperatures less than 550.degree. F. Since this blend does not upgrade the LCO's value significantly, there are only slight cost advantages, if any, with using LCO. Still further, LCO only has slightly more volumetric energy density than on-highway diesel fuel. LCO's API gravity is typically about 13.7 or higher, and its volumetric energy density is typically about 143,000 Btu/gallon or less. Thus, large amounts of LCO must be added to on-highway diesel fuel to increase its volumetric energy density substantially. The addition of large amounts of LCO sufficient to provide significant increases in volumetric energy density distorts the performance characteristics of the diesel fuel by greatly compromising its stability and increasing its potential to plug filter systems. Sufficient amounts of stabilizers cannot be added to the LCO and diesel fuel blend to make it stable. Thus, this LCO blend has not been found to be an acceptable diesel fuel alternative. Furthermore, many in the industry would expect that heavier cycle oil, when combined with on-highway diesel fuel, would have a greater tendency for sludge and gum formation because of its higher molecular weight and higher boiling point, and thus, it would be even less stable than LCO blends.
In order to overcome the deficiencies found with conventional fuels used in off-highway compression ignition engines, a fuel which can be used in all types of off-highway vehicles including both off-highway heavy duty and "very" heavy duty compression ignition engines and a method for making such fuel are needed. A fuel with a high volumetric energy density having acceptable compression ignition performance, a low amount of contaminants, and a cetane number and an API gravity optimized for off-highway use and a method for making such fuel in a cost effective manner are needed.